It is almost always caused by one or more brake rotors being warped. It's usually not that serious of a problem unless it vibrates really bad, or the front wheels want to lock up during a hard stop. You can repair the problem by replacing the front rotors or having them "turned" at a repair shop or auto parts store. The problem is, you can warp even a new rotor fairly easily. Hitting a cold puddle of water after a long hard stop or downhill braking, for instance, is all it takes.